Hi my name is Adam and I was wondering if you
know any formulas to aid in designing velocity stacks. I am going to
use them on a snowmobile but the theory should be the same for all two
strokes I want to make them resonate at around 8000 rpm. Thanks
for your help.

Adam,

I think what you are referring to is the sonic waves that occur on the
intake side of the motor. Velocity stacks change the tuning of the wave to
a broader rpm band. Unfortunately, all this is determined by engine
design, exhaust pipe design, intake design, bore/stroke ratios, rpm, and
just about everything else you can think of in this motor. Probably not
what you wanted to hear. Most of the snowmobile engines I have seen, are
tuned to work with the airbox the manufacturer designed for it. When you
remove the airbox, most of the time performance actually goes down. There
is a formula to figure a velocity stack at the rpm you are talking.
Whether it would make more power, remains to be seen and it would probably
be less than 1 or 2%. The book "Two Stroke Tuners Handbook" by
Gordon Jennings lists all of the formulas you are after. They are ungodly
lengthy, and complicated, but they are there nonetheless. If you would
like to make your velocity stacks work, I would suggest studying this
book. You will need to fill in all the blanks, as far as CC's and engine
design and such to come up with the right formula. But again, as you will
find in the book, the gains are not all that great. A good set of pipes on
your snowmobile will reap huge rewards, compared to the pitiful results
that the velocity stacks will give you back. Hope this gets you interested
in the book, as I think you will enjoy all the possibilities that it gives
you.

Randy

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